We have determined that capsular polysaccharide of a K54 clinical isolate of Escherichia coli is an important virulence factor. Capsular antigens in E. coli can be divided into two groups on the basis of microbiologic, physical and chemical characteristics. Group 1 capsules are associated with non-pathogens whereas, group 2 capsules are present in pathogenic extra-intestinal isolates. An understanding of group 2 capsular regulation may lead to novel therapeutic approaches, therefore we have undertaken studies in this area. Group 1 capsule regulators have been identified and cloned. We have examined by complementation, gene disruption and gene fusion experiments what effect they had on the group 2 capsules of our K54 clinical isolate. Genes for both group 1 and 2 capsular polysaccharide are present and are non-allelic. The group 1 positive regulators RcsA and B stimulate group 1 capsule production but apparently not group 2. A group 1 negative regulator, ion protease, either directly or indirectly decreases group 2 gene transcription. This decrease in group 2 gene transcription is associated with an increased serum sensitivity. This is consistent with the protective effect of group 2 capsule against the bactericidal effects of serum. We are presently in the process of identifying and cloning the group 2 regulators genes which appear to be, at least in part, separate from the group 1 capsular regulatory genes.